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I see you are listing off competitive sets. One thing you should know is that sharing a type does not instantly make the two pokemon competitively comparable. Milotic is completely different then the shell smash sweepers.

Carracosta and Cloyster are comparable though. Powerful offensive STAB and shell smash makes them both good rain team shell smasher, though skill link + icicle spear is usually so good and so powerful that Cloyster is generally better.

Carracosta can arguably function better on a sandstorm, though, since it gets a special defense boost.